How Does the Amount of Water Affect Photosynthesis?

Water affects photosynthesis in two fundamentally different ways: it serves as the raw material that gets split to release oxygen and electrons, and it acts as a gatekeeper that controls whether carbon dioxide can enter the leaf in the first place. When soil moisture drops, plants close microscopic pores on their leaves called stomata to conserve water, and that closure chokes off the supply of CO₂ needed for sugar production. The relationship is not a simple “more water, more photosynthesis” curve, though, because flooding can be just as damaging as drought.

Water as a Raw Ingredient

At the most basic level, water molecules are consumed during the light-dependent reactions of photosynthesis. Inside the chloroplast, a protein complex called photosystem II splits water into oxygen, hydrogen ions, and electrons. The oxygen exits through the stomata (it is the oxygen you breathe), and the electrons flow through a chain of reactions that ultimately help build sugars. This water-splitting step evolved only once in evolutionary history, in an ancestor of cyanobacteria, and it has been powering nearly all complex life ever since.¹ The enzyme responsible, photosystem II, uses a cluster of four manganese ions and one calcium ion to pry apart those water molecules.²

Even so, the sheer volume of water that a plant absorbs from the soil vastly exceeds what it actually feeds into photosystem II. Somewhere around 95–97% of the water a plant takes up through its roots is lost through transpiration, the process of water evaporating out of open stomata. Only a tiny fraction is chemically consumed in the light reactions. So the real bottleneck when water runs short is not that the plant runs out of raw material for photosystem II; the bigger issue is what happens at the stomata.

Stomatal Closure Is the First Domino

When soil starts drying out, plant roots sense falling water availability and trigger a hormonal alarm. The hormone abscisic acid (ABA) surges through the plant, initiating a cascade that causes guard cells surrounding each stoma to lose turgor and deflate, physically narrowing or shutting the pore.³ This is a survival tactic: a closed stoma loses far less water to the atmosphere. But it also blocks CO₂ from diffusing into the leaf, which starves the sugar-building machinery.

Research on drought responses has established that stomatal closure is the dominant limitation on photosynthesis during mild to moderate drought. As long as the drought stays in that range, the internal biochemistry of the leaf is still largely functional; the problem is simply a shortage of CO₂ getting in.⁴ If you could somehow pipe CO₂ directly into those leaves, photosynthesis would recover substantially. This matters for practical scenarios: a brief dry spell that causes stomata to close is usually reversible. Re-water the plant and the stomata reopen, CO₂ flows in again, and photosynthetic rates bounce back quickly.

When Drought Gets Severe, the Damage Goes Deeper

Push drought past the moderate stage and the problems go beyond a closed gate. Enzymes inside the chloroplast start to malfunction. The key enzyme responsible for fixing carbon, commonly called Rubisco, loses activity as leaf water content drops. In tobacco plants, researchers found that Rubisco activity correlated positively with leaf water content, and the decline was driven by the buildup of tightly bound inhibitor molecules on the enzyme rather than simple deactivation.⁵ Meanwhile, the supply of a molecule called RuBP, which Rubisco needs as its substrate, also drops. At severe drought, this metabolic impairment becomes the dominant bottleneck, overtaking stomatal closure.⁴

This two-stage breakdown explains why recovery from severe drought is slow compared with recovery from a brief dry spell. Once the biochemical machinery inside the chloroplast is damaged or inhibited, the plant has to rebuild or reactivate it, which takes time and energy.

The Danger of Light During Drought

Drought and bright sunlight make an especially punishing combination. Under normal conditions, a plant uses the energy from absorbed light to drive the water-splitting reactions and ultimately make sugar. But when stomata are closed and CO₂ is scarce, the plant cannot use all the light energy it is absorbing. That surplus energy has to go somewhere, and it generates reactive oxygen species (ROS), aggressive molecules that damage proteins, membranes, and DNA inside the chloroplast.⁶

This is called photoinhibition: the rate at which photosystem II gets damaged outpaces the rate at which the plant can repair it. The result is a measurable drop in the quantum yield of photosystem II, meaning the leaf gets less photosynthetic output per unit of light absorbed. Drought amplifies photoinhibition by restricting CO₂ fixation, which leaves even more excess light energy sloshing around with nowhere productive to go.⁶

Plants are not helpless against this, though. They have a safety valve called non-photochemical quenching (NPQ), which harmlessly dissipates excess light energy as heat before it can form those damaging molecules. Two key players in this system are the pigment zeaxanthin and a protein called PsbS. Under drought conditions, these two components work together more intensely, ramping up NPQ to protect the photosynthetic apparatus.⁷ Species differ in how effectively they deploy this defense. In a study of three laurel forest tree species, the one that produced the highest concentrations of zeaxanthin under drought showed the most effective photoprotection.⁸

Too Much Water Is Also a Problem

It is tempting to assume that if drought harms photosynthesis, then saturating a plant with water would maximize it. That assumption is wrong. Waterlogged soil is oxygen-starved soil, and roots need oxygen to function. When roots cannot respire properly, they lose their ability to take up water and nutrients, which sounds paradoxical: the roots are surrounded by water but cannot absorb it. The downstream effects include reduced stomatal conductance, reduced CO₂ assimilation, and lower overall photosynthetic rates.⁹

This is why overwatering a houseplant can produce wilting symptoms that look identical to underwatering. In both cases the leaf is losing turgor and the stomata are shutting down, just for different reasons. Flooding limits root metabolism due to soil oxygen deficiency, which in turn limits the root’s capacity to pull water into the plant even when the soil is saturated.¹⁰ In prolonged waterlogging, roots may die altogether, compounding the damage.

There is also a surface-level version of this problem. In very humid or foggy environments, a film of liquid water can coat leaf surfaces and physically block stomatal pores, preventing CO₂ from entering. Cloud-forest plants have evolved structural tricks to deal with this, such as clustering their stomata within narrow grooves surrounded by waxy rings that use surface tension to keep liquid water out of the pore, even when the leaf surface is drenched.¹¹

How Atmospheric Humidity Matters Separately from Soil Moisture

Soil water and atmospheric moisture affect photosynthesis through different mechanisms. Even when soil moisture is adequate, very dry air can suppress photosynthesis. The key factor is the vapor pressure deficit (VPD) between the inside of the leaf and the surrounding air. When VPD is high, meaning the air is much drier than the leaf interior, stomata tend to close to prevent excessive water loss. In castor bean, a species with high photosynthetic capacity under humid conditions, CO₂ assimilation dropped as VPD increased, across a temperature range from 20 to 40 degrees Celsius.¹²

This is why desert afternoons hit plants with a double punch: soil moisture is low and air humidity is low. Even a well-irrigated crop in a hot, dry climate can show a midday dip in photosynthesis because the leaf-to-air VPD triggers partial stomatal closure. Satellite-based monitoring has started using this afternoon depression signal in chlorophyll fluorescence data as an indicator of drought stress in dryland vegetation.¹³ In other words, the dip in photosynthesis that happens when stomata respond to dry air is large enough to be visible from orbit.

Hydraulic Failure and the Sequence of Collapse

Inside the plant, water travels from roots to leaves through xylem vessels under tension, like water in a straw you are sucking on. When drought intensifies, the tension in those vessels can become so extreme that dissolved gas comes out of solution and forms bubbles, a process called embolism. Those bubbles block the flow of water and can cause catastrophic hydraulic failure if enough vessels are affected.

An important finding from long-term observation of maize leaves during drought is that photosynthesis and transpiration decline substantially before embolism even begins.¹⁴ The stomatal closure and metabolic downregulation described in earlier sections serve as a safety margin: the plant sacrifices photosynthetic productivity to protect its plumbing. By the time embolism actually occurs, the leaf has already shut down most of its gas exchange. This is a deliberate trade-off, essentially the plant choosing to starve temporarily rather than risk permanent structural damage to its water transport system.

Why Some Plants Handle Low Water Better Than Others

Not all photosynthetic pathways are equally sensitive to water shortage. Plants that use the C4 pathway, like corn, sugarcane, and many tropical grasses, have a built-in advantage. C4 plants concentrate CO₂ around Rubisco using an additional biochemical step, which means they can keep photosynthesis running at high rates even with their stomata partially closed. Because they do not need their stomata open as wide, they lose less water per unit of carbon fixed, giving them higher water use efficiency than C3 plants like wheat, rice, and most trees.¹⁵

The practical difference is visible during progressive drought. In experiments comparing C3 and C4 species, photosynthesis in C3 plants declined soon after drought set in, while C4 plants maintained their rates through moderate drought before eventually declining under more severe stress.¹⁶ This is one reason why C4 grasses dominate hot, dry grasslands and savannas around the world.

Then there is the CAM pathway, used by cacti, agaves, and many succulents. CAM plants take the water-conservation logic even further: they open their stomata only at night, when temperatures are cooler and humidity is higher, to take in CO₂. They store it as an organic acid and then release it internally during the day to feed photosynthesis with stomata firmly closed. This is an extreme water-saving strategy, and it caps growth rates, but it allows photosynthesis to continue in environments where C3 and C4 plants would simply shut down. Recent research has even found low-level CAM activity in stressed carnivorous plants of the genus Pinguicula, suggesting this pathway can be induced by water deficit in plants not normally considered succulent.¹⁷

Mycorrhizal Fungi as Underground Allies

Plants do not face drought alone. The majority of land plants form partnerships with mycorrhizal fungi, thread-like organisms that colonize root tissue and extend far into the soil. The fungal network dramatically increases the effective surface area for water and nutrient absorption. Under drought, these partnerships become especially valuable.

In a field experiment comparing C3 and C4 grasses, plants colonized by arbuscular mycorrhizal fungi maintained significantly higher photosynthetic rates under light and moderate drought than uncolonized controls. In the C3 grass Leymus chinensis, fungal colonization boosted photosynthetic rate by about 63% and stomatal conductance by about 38% under drought. The C4 grass Hemarthria altissima also benefited, with about a 28% increase in photosynthetic rate.¹⁸ The fungi also reduced oxidative damage markers in the plant tissue, suggesting they help prevent the kind of ROS-driven injury described earlier. For anyone growing plants in dry conditions, encouraging healthy soil fungal communities through reduced tillage and limiting fungicide use can provide a genuine buffer against drought-related photosynthetic decline.

Rising CO₂ and Future Drought

Here is where the water-photosynthesis relationship intersects with climate change. Atmospheric CO₂ concentrations are rising, and that changes the math. Higher CO₂ in the air means a steeper concentration gradient from outside to inside the leaf, so more CO₂ can diffuse through even partially closed stomata. A global meta-analysis found that elevated CO₂ alleviated the stomatal limitations drought imposes on photosynthesis by increasing the CO₂ concentration inside the leaf.¹⁹

This sounds like good news, and in a narrow biochemical sense it is. Plants under moderate drought in a high-CO₂ world can maintain higher photosynthetic rates than they could under the same drought at lower CO₂. But the picture is complicated. Rising CO₂ is also driving the warming and altered precipitation patterns that make droughts more frequent and severe in many regions. And the benefit is strongest for C3 plants; C4 plants, which already concentrate CO₂ internally, gain less from the atmospheric increase.¹⁵ The net outcome for any given ecosystem depends on whether the CO₂ fertilization effect outpaces the increasing severity of water stress, and the evidence so far suggests it is a race with no clear winner across the board.

Practical Takeaways for Growers

Understanding the water-photosynthesis relationship changes how you should think about watering schedules. The goal is not to keep soil perpetually saturated. It is to keep soil moisture in a range where stomata stay open and roots stay oxygenated. For most plants, that range is well-drained soil that dries slightly between waterings. If the soil never dries, oxygen levels in the root zone drop and you get the same stomatal closure and reduced photosynthesis you would see from underwatering.¹⁰

Timing also matters. Watering in the morning, before the heat of the day drives VPD up, gives the plant a window of high stomatal conductance when soil moisture and atmospheric conditions are both favorable. In hot climates, afternoon stomatal closure is nearly unavoidable regardless of soil moisture, so front-loading the plant’s productive hours makes sense.

Mulching reduces soil evaporation and moderates soil temperature, extending the period between necessary waterings without letting root-zone moisture crash. For container plants and raised beds, the drainage profile of the soil mix matters more than the watering frequency: a heavy, poorly draining mix invites waterlogging, while a gritty, well-aerated mix lets you water generously without drowning roots. The real lesson from the science is that plants live on a knife edge between too little and too much, and photosynthesis is one of the first processes to tell you when you have crossed either line.

1
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2
PubMed Central. Photosystem II: the water-splitting enzyme of photosynthesis
3
PubMed Central. Mechanisms of abscisic acid-mediated control of stomatal aperture
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PubMed Central. Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal limitations revisited
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PubMed Central. Rubisco activity: effects of drought stress
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PubMed. The mechanisms of photoinhibition and repair in plants under high light conditions and interplay with abiotic stressors
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PubMed Central. Non-Photochemical Quenching under Drought and Fluctuating Light
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PubMed. Drought-induced oxidative stress in Canarian laurel forest tree species growing under controlled conditions
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PubMed. Xeromorphic traits help to maintain photosynthesis in the perhumid climate of a Taiwanese cloud forest
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PubMed Central. Control of Photosynthesis and Stomatal Conductance in Ricinus communis L. (Castor Bean) by Leaf to Air Vapor Pressure Deficit
13
Agricultural and Forest Meteorology. Tracking drought in dryland vegetation through the photosynthetic afternoon depression index of Sun-induced chlorophyll fluorescence
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PubMed Central. Long‐Term in vivo Observation of Maize Leaf Xylem Embolism, Transpiration and Photosynthesis During Drought and Recovery
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PubMed Central. Increasing water use efficiency along the C 3 to C 4 evolutionary pathway: a stomatal optimization perspective
16
HAYATI Journal of Biosciences. Photosynthesis of C3 and C4 Species in Response to Increased CO2 Concentration and Drought Stress
17
PubMed Central. Diel net CO(2) exchange of carnivorous Pinguicula is consistent with low-level CAM photosynthesis
18
PubMed Central. Arbuscular Mycorrhizal Fungi Alleviate Drought Stress in C3 (Leymus chinensis) and C4 (Hemarthria altissima) Grasses via Altering Antioxidant Enzyme Activities and Photosynthesis
19
Journal of Ecology. Elevated CO2 alleviates adverse effects of drought on plant water relations and photosynthesis: A global meta‐analysis